If you know the leak isnt the return *fitting*, then you should be able to get away with using threaded plugs. If the leak is from the fitting itself, youd need to remove and replace the whole fitting, rendering any threaded plugs (that go into the fitting) useless at stopping the water from leaking out.

I ended up draining my pool to check the return fittings for a leak last year, but it turned out the fitting was only a small part of a larger leak. Fixed the small part with the pool drained, filled the pool back up and used a threaded plug to replace the return plumbing.

Whatever water was in the return came out by itself when I disconnected it from the fitting at the pool. If you dig the trench first, you should be able to lift the plumbing up enough to prevent the water from flowing out. I just let it flow into the hole, because there wasnt much water. Or you can use a shop vac to try and blow it out.

-Chris